Currently, 3rd generation cellular communication systems are being rolled out to further enhance the communication services provided to mobile phone users. The most widely adopted 3rd generation communication systems are based on Code Division Multiple Access (CDMA) and Frequency Division Duplex (FDD) or Time Division Duplex (TDD) technology. In CDMA systems, user separation is obtained by allocating different spreading and/or scrambling codes to different users on the same carrier frequency and in the same time intervals. This is in contrast to time division multiple access (TDMA) systems, where user separation is achieved by assigning different time slots to different users.
In addition, TDD provides for the same carrier frequency to be used for both uplink transmissions, i.e. transmissions from the mobile wireless communication unit (often referred to as wireless subscriber communication unit) to the communication infrastructure via a wireless serving base station and downlink transmissions, i.e. transmissions from the communication infrastructure to the mobile wireless communication unit via a serving base station. In TDD, the carrier frequency is subdivided in the time domain into a series of timeslots. The single carrier frequency is assigned to uplink transmissions during some timeslots and to downlink transmissions during other timeslots. An example of a communication system using this principle is the Universal Mobile Telecommunication System (UMTS). Further description of CDMA, and specifically of the Wideband CDMA (WCDMA) mode of UMTS, can be found in ‘WCDMA for UMTS’, Harri Holma (editor), Antti Toskala (Editor), Wiley & Sons, 2001, ISBN 0471486876.
The Long Term Evolution (LTE) is a new terrestrial mobile communication standard currently being standardised by the 3GPP and is expected to be completed in the 2009/2010 timeframe. The Radio Access Network (RAN) of LTE is named as the Evolved-Universal Mobile Telecommunication Systems Radio Access Network (E-UTRAN). The E-UTRAN physical layer is based on Orthogonal Frequency Division Multiplexing (OFDM). More precisely; the downlink transmission scheme is based on conventional OFDM using a cyclic prefix while the uplink transmission is based on single-carrier frequency division multiple access (FDMA) techniques, more specifically DFTS-OFDM. The OFDM sub-carrier spacing is Δf=15 kHz in both uplink and downlink transmission. LTE supports both frequency division duplex (FDD) and time division duplex (TDD). More information on E-UTRAN standard can be found in TS 36.XXXX series of 3GPP document at: ftp://ftp.3gpp.org/Specs/latest/Rel-8/36_series/.
System Information (SI) in an LTE system is divided into a number of SystemInformationBlocks and MasterInformationBlock (MIB). The MIB includes a limited number of most essential and frequently transmitted parameters to acquire other information from the cell. SI is defined in [TS 36.300] as a RRC message carrying a number of SystemInformationBlocks that have the same periodicity. Each SystemInformationBlock contains a set of related system information parameters.
SystemInformationBlockType1 is transmitted alone, separately from other SI-messages. The MIB message is mapped on the Broadcast Control CHannel (BCCH) and carried on a Broadcast Channel (BCH). All other SI messages are mapped on the BCCH and carried on a DownLink Shared Channel (DL-SCH) where they can be identified through the SI-RNTI (System Information RNTI).
Also, SystemInformationBlocks other than SystemInformationBlocksType1 are carried in SI messages and mapping of SystemInformationBlocks to SI messages is flexibly configurable by using a schedulingInformation parameter included in SystemInformationBlocksType1, with restrictions that each SystemInformationBlock is contained only in a single SI message. Only SystemInformationBlocks having the same scheduling (periodicity) requirement can be mapped to the same SI message. SystemInformationBlocksType2 is always mapped to the SI message that corresponds to the first entry in the list of SI messages in the schedulingInformation parameter.
It is known that there may be multiple SI messages transmitted with the same periodicity. SystemInformationBlocksType1 and all SI messages are transmitted on DL-SCH.
The MIB uses a fixed schedule with a periodicity of 40 msec and repetitions made within 40 ms. The first transmission of the MIB is scheduled in subframe #0 of radio frames for which the SFN mod 4=0, and repetitions are scheduled in subframe #0 of all other radio frames. The SystemInformationBlockType1 uses a fixed schedule with a periodicity of 80 msec and repetitions made within 80 msec.
It is known that the first transmission of SystemInformationBlockType1 is scheduled in subframe #5 of radio frames for which the SFN mod 8=0, and repetitions are scheduled in subframe #5 of all other radio frames for which SFN mod 2=0.
The SI messages are transmitted within periodically occurring time domain windows (referred to as SI-windows) using a dynamic scheduling mechanism. Each SI message is associated with a SI-window and the SI-windows of different SI messages do not overlap. That is, within one SI-window only the corresponding SI is transmitted. The length of the SI-window is common for all SI messages, and is configurable. Within the SI-window, the corresponding SI message can be transmitted a number of times in any subframe other than subframes where SystemInformationBlockType1 is present (i.e. subframe #5 of radio frames for which SFN mod 2=0) amongst others.
SystemInformationBlockType1 configures the SI-window length and the transmission periodicity for the SI messages. A user equipment (UE) acquires the detailed time-domain scheduling (and other information, e.g. frequency-domain scheduling, information on the used transport format, etc.) from decoding the SI-RNTI on Physical Dedicated Control CHannel (PDCCH). A single SI-RNTI is used to address SystemInformationBlockType1 as well as other SI messages.
The UE acquires the system information upon selecting (e.g. upon power on) and upon re-selecting a cell, after handover completion, after entering E-UTRA from another radio access technology (RAT), upon return from an out-of-coverage area, upon receiving a notification that the system information has changed or upon exceeding the maximum validity duration of system information.
It is also known that the technical specification ‘TS 36.331’ specifies the system information acquisition procedure. A common procedure is defined for both frequency division duplex (FDD) and time division duplex (TDD) networks, with a common value range of SI-window length. The common procedure is defined to guarantee that SI-windows do not overlap in time, and that SI-windows are consecutive in the time domain. However, the calculation of SI-window and the starting point of each SI-window do not take into account the frame configuration (for example the downlink (DL)/uplink (UL) split) in a TDD network. As a consequence, the delivery of system information in TDD network is not possible with some of the configurations allowed in the standard.
Referring now to FIG. 1, the current LTE standard in TS 36.331 specifies the following procedure to determine the start of the SI-window for a particular SI message. For the concerned SI message, the known process commences with a determination of the number n, which corresponds to the order of entry in the list of SI messages. The list of SI messages is configured by schedulingInformation signalled in SystemInformationBlockType1, as shown in step 105. The known process then continues with a determination of the integer value x, in step 110, where:x=(n−1)*w, where w is the si-WindowLength. The value of SI-WindowLength is then signalled to the UE in SystemInformationBlockType1, as shown in step 115. One si-WindowLength is used for all the System Information Messages (SIs) configured in SystemInformationBlockType1.
The known process then calculates the respective subframe number within a radio frame, for example subframe #a, as shown in step 120, such that a=x mod 10. Subframe #a is the subframe which the SI-window for the concerned SI message is started. The known process then determines the radio frame of the single frequency network (SFN), as shown in step 120 such that:mod T=FLOOR(x/10),where: T is the SI-Periodicity of the concerned SI message.
The SI-window for the concerned SI message starts in subframe #a of the determined radio frame. Thereafter, the known process includes receiving a downlink (DL) Signalling CHannel (DL-SCH) using the SI-RNTI from the start of the SI-window. This continues, as shown in step 130 until the end of the SI-window whose absolute length in time is provided by si-WindowLength, or until the SI message was received, excluding the following subframes:                Subframe #5 in radio frames for which SFN mod 2=0        any multicast broadcast single frequency network (MBSFN) subframes;        any uplink subframes in the TDD mode of operation.        
If the SI message was not received by the end of the SI-window in step 130, the process of reception at the next SI-window occasion, for the concerned SI message, is repeated as shown in step 135.
According to the current agreement, 3 bits are used to signal si-WindowLength and take values of 1 msec, 2 msec, 5 msec, 10 msec, 15 msec, 20 msec, and 40 msec., with one spare, unallocated value. The maximum number of SI messages supported is ‘32’. The SI-periodicity can take a value in the range {80 msec, 160 msec, 320 msec, 640 msec, 1280 msec, 2560 msec, 5120 msec, Spare1}. Assuming eight SI messages are supported in the delivery of system information, the periodicity of SI-messages, for example, are as shown in Table 1.
TABLE 1periodicity of SI-messages signalled inSystemInformationBlockType1Order of the SI-messagePeriodicity [msec]SI-1160SI-2160SI-3160SI-4320SI-5640SI-6640SI-71280SI-82560
Assuming the SI-windowLength is 2 msec, the corresponding starting point of SI-window for above SI messages can be derived as in Table 2.
TABLE 2Starting point of SI-windows derived according to[TS 36.331] with si-WindowLength = 2 msec.Order of thePeriodicitySubframeRadio frameSI-message[ms]#anumberSI-116000, 16, 32, 48, 64SI-216020, 16, 32, 48, 64SI-316040, 16, 32, 48, 64SI-432060, 32, 64, 96, 128SI-564080, 64, 128, 192, 256SI-664001, 65, 129, 193, 257SI-7128021, 129, 257, 385, 513SI-8256041, 257, 513, 769, 1025
For a frequency division duplex (FDD) mode of operation, the transmission of SI messages at a subframe level 200 is shown in FIG. 2. It is noteworthy that the procedure specified in TS 36.331 guarantees non overlapping SI-windows for different SI-messages. The restriction in the use of subframe #5 is omitted in the following analysis, as the purpose of the analysis is to compare the differences between the operation in TDD and FDD networks. Referring now to FIG. 2, there are 7 different frame configuration types are defined in [TS 36.211] for TDD networks, which is represented below in Table 3.
TABLE 3The specified frame configuration types in known TDD networksaccording to [TS 36.211]subframe #0123456789Type 0DSUUUDSUUUType 1DSUUDDSUUDType 2DSUDDDSUDDType 3DSUUUDDDDDType 4DSUUDDDDDDType 5DSUDDDDDDDType 6DSUUUDSUUDwhere:
D represents a downlink subframe,
U represents an uplink subframe and
S represents a special frame, such that an ‘S’ frame can be used in both DL and UL transmission.
Considering the frame configuration type0 in Table 3, the corresponding starting points 300 of SI-windows in a TDD mode of operation is shown in FIG. 3 for an SI-WindowLength=2 msec. As shown in FIG. 3, an SI-window corresponding to an SI-2 message has entirely overlapped with uplink subframes. Hence, transmission of an SI-2 message is blocked. Similarly, transmission of SI-5 and SI-7 messages is also blocked by the presence of UL subframes. Furthermore, the transmission windows of SI-3, SI-4 and SI-8 messages are partially blocked by the UL subframes.
In this example of the known subframe configuration, transmission of SI-2, SI-3, SI-5 and SI-7 messages are not possible in a TDD network if the SI acquisition procedure as defined in [TS 36.331] is employed. The SIs are numbered according to the order of their appearance of signalling in SystemInfromationBlockType1. It is not possible to signal a null (empty) SI message in SystemInfromationBlockType1. Therefore, the transmission of SI messages which take the order 2, 3, 5, or 7 are blocked by the presence of TDD UL subframes. Thus, these messages are unable to be delivered to the UE. Hence, as the UE is not able to receive the required system information, the UE operation in TDD system is not possible.
A further illustration that considers the TDD frame configuration type1 400 is shown in tabular form in FIG. 4. The SI message configuration is as shown in Table 2.
Here, it can be seen that the SI-2 and SI-7 messages are entirely blocked by the UL subframes, while SI-4 and SI-5 messages are also partially blocked.
Furthermore, it is worth considering the SI window configurations for TDD frame configuration type 3, 4 and 6, which are illustrated in FIGS. 5A, 5B and 5C for the SI-message configuration provided in Table 2. Here, in all three illustrated cases 500, 510 and 520, a successful reception of transmitted SI-2 and SI-7 messages is not possible according to the currently specified procedure in [TS 36.331].
FIGS. 6A and 6B illustrate the SI-message configuration for TDD frame configuration type2 600 and type5 610. Here, it is clearly shown that the transmission of SI-messages is not entirely blocked by the presence of UL subframes in these two frame configurations. However, the SI-message transmission is partially blocked in SI-2, SI-4 and SI-7 messages in type2. Furthermore, SI-2 and SI-7 messages are partially blocked in type5
Thus, the above analysis illustrates that the operation of the currently specified SI acquisition procedure in TS 36.331, when applied to a TDD network with different frame configurations, is unacceptable. As explained, transmission of some SI messages is impossible with some of the TDD frame configurations. In other cases, the SI-window is partially blocked by the presence of UL subframes. Thus, the outcome of the specified procedure is different depending on the frame configuration and, hence, does not provide an accurate procedure in a TDD network.
A yet further example of the problems associated with the current LTE frame configuration is shown in FIG. 7, for a case where there is a 1 msec SI-WindowLength 700. The periodicity of SI message are as shown in Table 4 and the configuration of SI-windows is as shown in Table 5.
TABLE 4periodicity of SI-messages signalled inSystemInformationBlockType1Order of the SI-messagePeriodicity [msec]SI-116SI-232SI-364SI-432SI-532SI-664SI-7128SI-8256SI-9128
TABLE 5starting point of SI-windows derived according to[TS 36.331] with SI-WindowLength = 1 msecOrder of thePeriodicitySubframeSI-message[msec]#aRadio frame numberSI-11600, 16, 32, 48, 64SI-23210, 32, 64, 96, 128SI-36420, 64, 128, 192, 256SI-43230, 32, 64, 96, 128SI-53240, 32, 64, 96, 128SI-66450, 64, 128, 192, 256SI-712860, 128, 256, 384, 512SI-825670, 256, 512, 768,1024SI-912880, 128, 256, 384, 512
As shown in the above Tables, the transmission of SI-3 and SI-8 messages is blocked in radio configuration type0, type1 and type2, while the transmission of SI-4 and SI-9 messages is blocked in radio configurations type0 and type 2. It is also noteworthy that the SI-5 message is also blocked in type0.
Furthermore, as illustrated with respect to FIG. 3 and Table 6, the specified procedure in the current standard [TS 36.331] does not provide an accurate mechanism for transmission of SI-messages in TDD networks.
A yet further problem in the specified procedure is illustrated below, by considering a 5 msec SI-WindowLength with SI message periodicity as given in Table 1. Table 6 lists the calculated starting points of SI-messages according to the specified procedure in [TS 36.331].
TABLE 6Starting point of SI-windows derived according to[TS 36.331] with SI-WindowLength = 5 msecOrder of thePeriodicitySubframeSI-message[msec]#aRadio frame numberSI-116000, 16, 32, 48, 64SI-216050, 16, 32, 48, 64SI-316001, 17, 33, 49, 65SI-432051, 33, 65, 97, 129SI-564002, 66, 130, 194, 258SI-664052, 66, 130, 194, 258SI-7128003, 131, 259, 387, 515SI-8256053, 259, 515, 771, 1027
The resulting SI window configuration for TDD frame configurations in Type3 800 and Type4 810 are shown in FIGS. 8A and 8B respectively. As shown in FIG. 8 the transmission of SI-messages is not entirely blocked due to the presence of UL subframes. However, the effective SI-WindowLength for SI-1, SI-3, SI-5 and SI-7 messages has reduced to 2 msec, while the effective SI-WindowLength for SI-2, SI-4, SI-6 and SI-8 messages remains at 5 msec.
As mentioned earlier, SI-messages are repeated at every subframe (except TDD UL subframes) within the corresponding SI-window. In this example SI-1, SI-3, SI-5 and SI-7 messages can only be repeated twice while SI-2, SI-4, SI-6 and SI-8 messages can be repeated five times within the same SI-window.
The UE is able to combine the received copies of the same message at the decoder, thereby increasing the reliability of the reception. The known combining of these messages at the UE is similar to the combining method used in Hybrid Automatic Repeat reQuest (HARQ) techniques. However, in this case the message is re-transmitted a fixed number of times, which is allowed by the effective SI-WindowLength. Nevertheless, the re-transmission is not based on the feedback from the UE as in normal HARQ operation.
According to the example shown above, transmission of SI-2, SI-4, SI-6 and SI-8 messages are more robust than the transmission of SI-1, SI-3, SI-5 and SI-7 messages due to their number of repetitions with in the SI-window. However, all the system information messages should be transmitted with the same robustness as all the system information messages are equally important for the correct operation of the UE. Unequal reception reliability of the SI-messages are not seen in FDD networks as the SI-window length is defined as the number of re-transmissions of each message, and it is the same for all the SI-messages with the exception of SI-messages that are mapped onto subframe #5 of even radio frames.
Consequently, current techniques are suboptimal. Hence, an improved mechanism to address the problems of acquiring a system information message over a cellular network such as an LTE-TDD network would be advantageous.